Most of these devices have a hardware bug in the ethernet chip. This leads to connection loss of eth0 interface. Only a reboot can fix this. See also Bug description. As of August 2015 there is no workaround in OpenWrt available.

From the beginning of 2014 Ubiquiti has released new model of Nanostation M5 devices (old boards use ar724x chip while new ones are using ar934x chip).